1. Field of the Invention
The invention relates to a forced once-through steam generator having an evaporator heating surface, a device connected upstream of the evaporator heating surface in terms of flow for setting a feed-water mass flow M into the evaporator heating surface, and a control device being assigned to the device, having a control variable being the feed-water mass flow M and having a setpoint value M.sub.s for the feed-water mass flow being controlled as a function of a setpoint value L assigned to the steam generator power.
Such a forced once-through steam generator is disclosed in the publication entitled: "VGB Kraftwerkstechnik 65", No 1 January 1985, page 29, FIG. 6. In that known forced once-through steam generator, in order to synchronize the heat flow into the evaporator heating surface with the feed-water mass flow, the setpoint value for the feed-water mass flow is controlled by the setpoint value of the steam generator power or by a setpoint value assigned to the steam generator power, through a delay element. Other measures are not provided for that synchronization.
It has been found that in that known forced once-through steam generator, an overshoot of the specific enthalpy at the outlet of the evaporator heating surface cannot be avoided when the steam generator power changes as a consequence of load changes. Such an overshoot may not only reduce the service life of the once-through steam generator but may also hamper the control of the temperature of the live steam delivered by the once-through steam generator.
2. Summary of the Invention
It is accordingly an object of the invention to provide a forced once-through steam generator, which overcomes the hereinafore-mentioned disadvantages of the known devices of this general type and which substantially reduces or avoids completely the disadvantageous overshoot of the specific enthalpy at the outlet of the evaporator heating surface.
With the foregoing and other objects in view there is provided, in accordance with the invention, a forced once-through steam generator, comprising an evaporator heating surface having an inlet and an outlet; a device connected upstream of the evaporator heating surface in terms of flow for setting a feed-water mass flow M into the evaporator heating surface; a control device being associated with the device and having a control variable being the feed-water mass flow M and a setpoint value M.sub.s for the feed-water mass flow being controlled as a function of a setpoint value L assigned to a steam generator power; another device associated with the control device for deriving a variable Q(L1)/(h.sub.sA (L2)-h.sub.iE) as the setpoint value M.sub.s for the feed-water mass flow, the other device receiving an actual value h.sub.iE of a specific enthalpy at the inlet of the evaporator heating surface and the setpoint value L assigned to the steam generator power, as input variables; a function generator from which a value Q(L1) for a heat flow into the evaporator heating surface is derived by a first power value L1, in accordance with a function of the first power value L1 to be fixedly predetermined; a setpoint value h.sub.sA (L2) for a specific enthalpy at the outlet of the evaporator heating surface being derived by a second power value L2 from the function generator in accordance with a function of the second power value L2 to be fixedly predetermined; a first delay element delaying the first power value L1 relative to the setpoint value L assigned to the steam generator power; and a second delay element delaying the second power value L2 relative to the first power value L1.
The processing of the actual value of the specific enthalpy at the inlet of the evaporator heating surface makes it possible to use the heat flow flowing into the evaporator heating surface to determine the setpoint value for the feed-water mass flow, with the result that the feed-water mass flow fed to the evaporator heating surface can largely be matched to the heat flow fed to the evaporator heating surface. This permits a systematic control of the specific enthalpy at the outlet of the evaporator heating surface.
In accordance with another feature of the invention, the device for deriving the variable Q(L1)/(h.sub.sA (L2)-h.sub.iE)=M.sub.s includes a differentiating element having an input being connected to the second power value L2 at the output of the second delay element or to the actual value of a pressure measured downstream of the evaporator heating surface and temporarily reducing the value of the variable derived as a setpoint value M.sub.s by a correction value, if the second power value L2 at the output of the second delay element or of the actual value of the pressure measured downstream of the evaporator heating surface rises, and temporarily increasing it by a correction value if the second power value L2 or the actual value of the pressure measured downstream of the evaporator heating surface decreases. This allows for energy storage in the metal masses of the evaporator heating surface, with the result that the feed-water mass flow fed to the evaporator heating surface is even better matched to the heat flow being fed to the evaporator heating surface.
In accordance with a further feature of the invention, the device for deriving the variable Q(L1)/(h.sub.sA (L2)-h.sub.iE)=M.sub.s includes a functional element with a differentiating characteristic, having an input being connected to the actual value h.sub.iE of the specific enthalpy at the inlet of the evaporator heating surface and temporarily reducing the value of the variable derived as a setpoint value M.sub.s by a correction value if the actual value h.sub.iE of the specific enthalpy at the inlet of the evaporator heating surface rises and temporarily increasing it by a correction value if the actual value h.sub.iE decreases. This takes account of the fact that the effects of mass flow and temperature changes of the feed water entering the evaporator heating surface do not proceed synchronously in the evaporator heating surface.
In accordance with an added feature of the invention, there is provided an enthalpy correction control having a controller input for receiving the variable (h.sub.sA (L2)-h.sub.iA) as a control deviation and having a controller output for supplying a correction value being added to a difference (h.sub.sA (L2) h.sub.iE), where h.sub.iA is the actual value of the specific enthalpy at the outlet of the evaporator heating surface.
In accordance with an additional feature of the invention, there is provided a multiplication element, the function generator including a first and a second function generator unit receiving the first power value L1 and supplying output signals (M(L1), .DELTA.h(L1) being fed to the multiplication element.
In accordance with yet another feature of the invention, there is provided a summing element, the function generator including a third function generator unit receiving the second power value L2 and supplying an output signal (h.sub.sA (L2)) to be fed to the summing element.
In accordance with yet a further feature of the invention, the other device includes a dividing element for deriving the variable M.sub.s.
In accordance with a concomitant feature of the invention, there is provided a measuring device for determining the actual value of the specific enthalpy at least at one of the inlet and the outlet of the evaporator heating surface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a forced once-through steam generator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.